"I am confident that these countries can also send a message that we won't let anyone take Europe from us," he said heading into a meeting in Berlin of his counterparts from the EU's six founding members.
His French counterpart Jean-Marc Ayrault urged quick negotiations on Britain's exit from the union, saying that the pressure would be "very strong" on British Prime Minister David Cameron at an EU summit Tuesday to speed up the process.
Cameron, who Friday announced his resignation by October in the wake of the referendum, said it should be his successor who leads the complex negotiations under Article 50 of the EU's Lisbon Treaty which sets out a two-year timeframe to leave.
Steinmeier called the European Union "a successful project of peace and stability" and said that there was a "strong desire" within the bloc to defend and strengthen it.
"I think it is absolutely clear that we are in a situation in which neither hysteria nor paralysis are permissible," he said, AFP reported.
"We must not rush headlong into hectic action, pretending we had all the answers. But we must also not fall into depression or inaction after the British decision."
Steinmeier hosted Ayrault, the Netherlands' Bert Koenders, Italy's Paolo Gentiloni, Belgium's Didier Reynders and Luxemburg's Jean Asselborn in a lakeside villa north of the city center.
Ayrault said he and Steinmeier, whose countries long represented the twin-engine of European integration, were working on joint proposals that could deepen cooperation among EU members that use the euro currency, or bolster security and defense coordination.
Steinmeier said the ministers would discuss joint action on the refugee influx, the unemployment crisis and security during their meeting scheduled to wrap up around 1000 GMT with a news conference.